The Global Positioning System (GPS) constellation of satellites transmits a 10-Watt signal from space. The power level of the signal on earth is -166 dBW or about 20 dB below the radio frequency (RF) L-band noise floor of a typical earth environment. Since the GPS signals are spread-spectrum modulated, the process gain of 43 dB (for L2 band) is sufficient to recapture the low-power signal. However, GPS bands are subject to interference from cell phones, television broadcast harmonics as well as the intentional jamming thereof. Because of the widespread adoption of GPS for military and commercial navigation, inadvertent interference as well as jamming from hostile parties represents a potentially serious problem.
Several antenna concepts have been developed to mitigate interference for GPS receivers. For example, controlled-radiation pattern antennas (CRPA) are currently being considered in several weapons systems as an anti-jam element. The CRPA operates using spatial filtering techniques. More specifically, assuming a stable platform and constant radiation, a CRPA drives an antenna null in a direction of received interference. However, drawbacks associated with CRPA's include it's cost, variable convergence delay, aerodynamic penalty, radar cross-section and performance limits.
As an alternative to antenna-based anti-jam systems and methods, classical digital signal processing (DSP) can be used to discriminate signals in the electromagnetic spectral frequency. Classical DSP (e.g., uniform scale Fourier Transform) is best applied to stationary signals or those signals having constant statistical properties. However, if the interference is dynamic (i.e., non-stationary), neither classical DSP techniques nor CRPA's perform well.